The tumor suppressor activity of the BRCA1 protein can be overridden by growth promoter cyclin D1 in breast cancer cells, because both compete for the receptor for estrogen. By preventing estrogen to bind at the receptor site, BRCA1 interferes with estrogen?s growth promoting signal. However, due to the abundance of cyclin D1 in cancer cells, it preempts BRCA1 binding and promotes cell proliferation.
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